The most distinct features of gram positive bacteria are the presence of lipoteichoic acid and a membrane system consisting of a plasma membrane and mesosomes. The purpose of this project is to relate membrane structure with function using Staphylococcus aureus as the model test system. The major emphasis is to define the function (or functions) of mesosomes which contain most of the lipoteichoic acid in the cell. To support the chemical localization of lipoteichoic acid in mesosomes by immunochemical methods, antiserum to lipoteichoic acid was prepared. The activity and specificity of this antiserum to lipoteichoic acid was characterized by quantitative precepitin, inhibition, and immunoelectrophoretic studies while the localization of this polymer in mesosomes was ascertained by passive hemoagglutination inhibition tests. Also, lipoteichoic acid was shown for the first time to be an activator of Streptolysin S produced by Group A streptococci.